I've never understood the desire to do juice fasting. The insulin surges alone would keep me from even trying it. According to experts, juices don't belong in a healthy Diet even if you're not fasting. It's liquid calories and causes blood Sugar spikes. I guess there are "non-caloric" juices out there (flavored waters or whatever???) but by definition if you're drinking non-caloric liquids you're water fasting, not juice fasting.
Do what you want, though. I think I read that what you can lose in a week on water will take you a month to lose on juice. But just understand juice isn't health food to begin with.